Android DefualtTaskExecutor caching common TaskRunners

Bug: 877963
Change-Id: I6990e89e02de0a16800d0d29311a91b646c145a5
Reviewed-on: https://chromium-review.googlesource.com/c/1423318
Commit-Queue: Sam Maier <smaier@chromium.org>
Auto-Submit: Sam Maier <smaier@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624628}
2 files changed